feat(pwa): implement real offline mode with IndexedDB sync

Add genuine offline read/write capability for trips:

- Dexie IndexedDB schema (trips, places, packing, todo, budget,
  reservations, files, mutationQueue, syncMeta, blobCache)
- Repo layer for all domains: offline reads from Dexie, writes
  optimistically to Dexie and enqueue mutations for later replay
- Mutation queue with UUID idempotency keys (X-Idempotency-Key),
  FIFO flush, temp-ID reconciliation on 2xx, fail-and-continue on 4xx
- Trip sync manager: caches all trips with end_date >= today or null,
  auto-evicts 7d after end_date, fetches bundle endpoint in one request
- Map tile prefetcher: bbox from place coords, zooms 10-16, 50MB cap,
  warms SW cache via fetch
- Sync triggers: network online → flush + syncAll; WS reconnect →
  flush only (rate-limiter safe); visibilitychange/30s → flush only
- WS remoteEventHandler writes through to Dexie on every event
- Server idempotency middleware + idempotency_keys table (migration 100,
  24h TTL nightly cleanup)
- GET /api/trips/:id/bundle endpoint for efficient single-request sync
- OfflineBanner component: amber (offline) / blue (syncing) / hidden
- OfflineTab in Settings: cached trip list, re-sync and clear actions
- usePendingMutations hook for per-item pending indicators

Closes #505 #541

client/src/store/tripStore.ts

@@ -1,6 +1,11 @@
 import { create } from 'zustand'
 import type { StoreApi } from 'zustand'
-import { tripsApi, daysApi, placesApi, packingApi, todoApi, tagsApi, categoriesApi } from '../api/client'
+import { tripsApi, tagsApi, categoriesApi } from '../api/client'
+import { tripRepo } from '../repo/tripRepo'
+import { dayRepo } from '../repo/dayRepo'
+import { placeRepo } from '../repo/placeRepo'
+import { packingRepo } from '../repo/packingRepo'
+import { todoRepo } from '../repo/todoRepo'
 import { createPlacesSlice } from './slices/placesSlice'
 import { createAssignmentsSlice } from './slices/assignmentsSlice'
 import { createDayNotesSlice } from './slices/dayNotesSlice'
@@ -78,19 +83,19 @@ export const useTripStore = create<TripStoreState>((set, get) => ({
 
   setSelectedDay: (dayId: number | null) => set({ selectedDayId: dayId }),
 
-  handleRemoteEvent: (event: WebSocketEvent) => handleRemoteEvent(set, event),
+  handleRemoteEvent: (event: WebSocketEvent) => handleRemoteEvent(set, get, event),
 
   loadTrip: async (tripId: number | string) => {
     set({ isLoading: true, error: null })
     try {
       const [tripData, daysData, placesData, packingData, todoData, tagsData, categoriesData] = await Promise.all([
-        tripsApi.get(tripId),
-        daysApi.list(tripId),
-        placesApi.list(tripId),
-        packingApi.list(tripId),
-        todoApi.list(tripId),
-        tagsApi.list(),
-        categoriesApi.list(),
+        tripRepo.get(tripId),
+        dayRepo.list(tripId),
+        placeRepo.list(tripId),
+        packingRepo.list(tripId),
+        todoRepo.list(tripId),
+        tagsApi.list().catch(() => ({ tags: [] })),
+        categoriesApi.list().catch(() => ({ categories: [] })),
       ])
 
       const assignmentsMap: AssignmentsMap = {}
@@ -121,7 +126,7 @@ export const useTripStore = create<TripStoreState>((set, get) => ({
 
   refreshDays: async (tripId: number | string) => {
     try {
-      const daysData = await daysApi.list(tripId)
+      const daysData = await dayRepo.list(tripId)
       const assignmentsMap: AssignmentsMap = {}
       const dayNotesMap: DayNotesMap = {}
       for (const day of daysData.days) {
@@ -138,7 +143,7 @@ export const useTripStore = create<TripStoreState>((set, get) => ({
     try {
       const result = await tripsApi.update(tripId, data)
       set({ trip: result.trip })
-      const daysData = await daysApi.list(tripId)
+      const daysData = await dayRepo.list(tripId)
       const assignmentsMap: AssignmentsMap = {}
       const dayNotesMap: DayNotesMap = {}
       for (const day of daysData.days) {